1. Field of Invention
The invention generally relates to position estimates by a positioning system, and, more specifically, to the identification and/or quantification of features of positioning system reference points that indicate the reference points could yield unreliable and/or deceptive positioning information, and, in some cases, should not be relied upon for positioning information.
2. Description of Related Art
In recent years, mobile and portable communication and computing devices have become ubiquitous, and wireless communication systems have expanded to meet the corresponding demand for connectivity. Mobile and portable devices have no fixed locations and often accompany their users as they move, and application developers have seized on the opportunity to create applications that update or adapt based on the locations of the mobile devices running the applications. Intuitive examples include maps that update to indicate the current position of the device or advertisements that adapt based on proximity to a particular point of interest.
In order to enable location-aware applications, device makers must make their devices capable of acquiring position information with minimal user input. On the regulatory front, the FCC enhanced 911 rules mandate that mobile telephones must be able to supply location information to emergency operators when making 911 calls. One conventional solution to the problem of device positioning is GPS, which uses transmissions from satellites that follow carefully prescribed orbits. Unfortunately, GPS often fails to cover indoor and densely developed urban areas, requires dedicated hardware, and often suffers from slow time to first fix. Thus, more and more positioning systems are based on “beacons of opportunity” such as IEEE 802.11 access points and cellular base stations. They use transmissions from existing wireless communication systems that are received by the standard hardware of the mobile device and combine them with known information about the beacons to determine the position of the mobile device. Such systems effectively complement GPS coverage areas while providing fast time to first fix and requiring no dedicated hardware.
There are currently numerous beacon-based positioning systems both in the research community and in commercial and industrial deployments, and they can be broadly divided into those that use pattern matching (also known as fingerprinting) and those that use distance estimates to known reference points. Pattern matching positioning technologies as developed by Placelab and others (“Practical Metropolitan-Scale Positioning for GSM Phones”, Chen et al.) estimate the position of the client device by matching its observations to a map of received signal values collected in the area.
In contrast, systems that use distance estimates explicitly estimate beacon locations rather than simply mapping the patterns of received signal strength from the beacons. Such systems then estimate the range from the client device to the observed beacons based either on signal propagation time or received signal strength (RSS).
Time-based systems use measurements of the time between transmission and reception of a signal to estimate the distance between the transmitter and the receiver. Such systems employ time of arrival (TOA) or time-difference of arrival (TDOA) schemes to generate range estimates for use in a variety of algorithms to generate position estimates for the user (US RE38,808, Schuchman, et al; US 2002/007715 A1, Ruutu, et al). However, in asynchronous systems such as GSM and UMTS, additional equipment is often installed at each cell at significant additional cost (U.S. Pat. No. 6,275,705 B1, Drane, et al; U.S. Pat. No. 6,526,039 B1, Dahlman, et al.; U.S. Pat. No. 6,901,264 B2, Myr).
Systems that use received signal strength (RSS) to estimate the distance from the mobile to the transmitting beacon use the fact that RSS is strongly related to the distance from the transmitter to the receiver (“Indoor/Outdoor Location of Cellular Handsets Based on Received Signal Strength” by Zhu and Durgin). Well-known pathloss models show that signal power falls exponentially with distance, so knowledge of the pathloss exponent and other parameters such as antenna gain and transmit power allows the positioning system to compute range estimates. Several well-known beacon-based positioning systems use this approach, notably in the form of wi-fi positioning (WPS) based on IEEE 802.11 access points.